Pollution control devices typically comprise a metal housing with a monolithic element securely mounted within the casing by a resilient and flexible mounting mat. Pollution control devices are universally employed on motor vehicles to control atmospheric pollution. Generally the pollution control device is designed according to the type of exhaust gas to be treated because the composition of the exhaust as well as temperatures thereof may be different depending on the type of engine causing the exhaust. Accordingly, pollution control devices are known to be used to treat the exhaust of gasoline engines as well as diesel engines. Pollution control devices include catalytic converters and particulate filters or traps. Two types of devices are currently in widespread use—catalytic converters and diesel particulate filters or traps. Catalytic converters contain a catalyst, which is typically coated on a monolithic structure mounted within a metallic housing. The monolithic structures are typically ceramic, although metal monoliths have also been used. The catalyst oxidizes carbon monoxide and hydrocarbons and reduces the oxides of nitrogen in automobile exhaust gases to control atmospheric pollution.
Diesel particulate filters or traps are typically wall flow filters, which have honeycombed, monolithic structures typically made from porous crystalline ceramic materials. Alternate cells of the honeycombed structure are typically plugged such that exhaust gas enters in one cell and is forced through the porous wall to an adjacent cell where it can exit the structure. In this way, the small soot particles that are present in diesel exhaust gas are collected.
The monoliths and in particular the ceramic pollution control monoliths, used in pollution control devices are fragile and susceptible to vibration or shock damage and breakage. They have a coefficient of thermal expansion generally an order of magnitude less than the metal housing which contains them. This means that as the pollution control device is heated the gap between the inside peripheral wall of the housing and the outer wall of the monolith increases. Likewise, as the temperature of the pollution control device drops (e.g., when the engine is turned off), this gap decreases. Even though the metallic housing undergoes a smaller temperature change due to the insulating effect of the mat, the higher coefficient of thermal expansion of the metallic housing causes the housing to expand to a larger peripheral size faster than the expansion of the monolithic element. This higher coefficient of thermal expansion also causes the metal housing to shrink to a smaller peripheral size faster than the monolithic element. Thermal cycling and these resulting physical changes can occur hundreds or even thousands of times during the life and use of the pollution control device.
To avoid damage to pollution control elements such as ceramic monoliths (e.g., from road shock and vibrations), to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and metal housing (thereby bypassing the catalyst and/or filter), mounting mats are disposed between the pollution control element and the housing. These mats must exert sufficient pressure to hold the pollution control element in place over the desired temperature range but not so much pressure as to damage the pollution control element (e.g., a ceramic monolith).
Many of the mounting mats described in the art have been developed for mounting the catalyst carrier of catalytic converters for treatment of exhaust from gasoline engines which typically operate at high temperature. Known mounting mats include intumescent sheet materials comprised of ceramic fibers, intumescent materials and organic and/or inorganic binders. Intumescent sheet materials useful for mounting a catalytic converter in a housing are described in, for example, U.S. Pat. No. 3,916,057 (Hatch et al.), U.S. Pat. No. 4,305,992 (Langer et al.) U.S. Pat. No. 5,151,253 (Merry et al.) U.S. Pat. No. 5,250,269 (Langer) and U.S. Pat. No. 5,736,109 (Howorth et al.). In recent years, non-intumescent mats comprised of polycrystalline ceramic fibers and binder have been used especially for the so-called ultra thin-wall monoliths, which have significantly lower strength due to their extremely thin cell walls. Examples of non-intumescent mats are described in, for example, U.S. Pat. No. 4,011,651 (Bradbury et al.), U.S. Pat. No. 4,929,429 (Merry), U.S. Pat. No. 5,028,397 (Merry), U.S. Pat. No. 5,996,228 (Shoji et al.), and U.S. Pat. No. 5,580,532 (Robinson et al.). Polycrystalline fibers are much more expensive than normal, melt formed ceramic fibers and, therefore, mats using these fibers are only used where absolutely necessary as, for example, with ultra thin-wall monoliths.
U.S. Pat. No. 5,290,522 describes a catalytic converter having a non-woven, mounting mat comprising at least 60% by weight shot-free high strength magnesium aluminosilicate glass fibers having a diameter greater than 5 micrometers. The mounting mats taught in this reference are primarily intended for use in high temperature applications as can be seen from the test data in the examples where the mats are subjected to exhaust gas temperatures of more than 700° C.
U.S. Pat. No. 5,380,580 describes a flexible non-woven mat comprising shot-free ceramic oxide fibers selected from the group consisting of (a) aluminosilicate fibers comprising aluminum oxide in the range from 60 to about 85% by weight and silicon oxide in the range of 40 to about 15% by weight silicon oxide, based on the total weight of said aluminosilicate—based fibers, said aluminosilicate-based fibers being at least 20% by weight crystalline (b) crystalline quartz fibers and (c) mixtures of (a) and (b), and wherein the combined weight of said aluminosilicate-based fibers and said crystalline quartz fibers is at least 50% by weight of the total weight of said non-woven mat. The flexible non-woven mat can additionally comprise high strength fibers selected from the group consisting of silicon carbide fibers, silicon nitride fibers, carbon fibers, silicon nitride fibers, glass fibers, stainless steel fibers, brass fibers, fugitive fibers, and mixtures thereof.
Diesel Oxidation Catalysts (DOC's) are used on modern diesel engines to oxidize the soluble organic fraction (SOF) of the diesel particulate emitted. Because of the relatively low exhaust gas temperatures, mounting of DOC's with conventional mounting materials has been problematic. The exhaust gas of modern diesel engines such as turbo-charged direct injection (TDI) engines may never exceed 300° C. This temperature is below the temperature needed to expand most intumescent mats. This expansion is needed to develop and maintain appropriate pressure within the catalytic converter.
U.S. Pat. No. 6,231,818 attempts to overcome the present difficulties of mounting low-temperature, diesel catalysts by using non-intumescent mats comprised of amorphous, inorganic fibers. Although it is taught in this patent that the mat can be organic binder free, it appears that several of the mats used in the examples require the use of substantial amounts of binders. Moreover, it was found that the mounting mats disclosed in this US patent, still do not adequately perform for treatment of exhaust from diesel engines, in particular TDI engines.
EP 1388649 discloses a pollution control device suitable for use with a diesel engine, comprising a diesel pollution control monolith arranged in a metallic casing with non-woven mat disposed between the metallic casing and the diesel pollution control monolith. The non-woven mat is a non-intumescent mat comprising at least 90% by weight based on the total weight of the mat of chopped magnesium aluminium silicate glass fibers that have a number average diameter of 5 μm or more and a length of 0.5 to 15 cm and the glass fibers are needle punched or stitch bonded and the mat being free or substantially free of organic binder.